Homeothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This temperature is often, though not necessarily, higher than the immediate environment (Greek: homoios = "similar", thermē = "heat"). The opposite is poikilothermy.

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Enzymes have a relatively narrow temperature range at which their efficiencies are optimal. Temperatures outside this range can greatly reduce the rate of a reaction or stop it altogether. A creature with a fairly constant body temperature can therefore specialize in enzymes which are efficient at that particular temperature. Another advantage of a homeothermic animal is its ability to maintain its constant body temperature even in cold weather. A poikilotherm must either operate well below optimum efficiency most of the time, migrate, hibernate or expend extra resources producing a wider range of enzymes to cover the wider range of body temperatures.

However, some environments offer much more consistent temperatures than others. For example, the tropics often have seasonal variations in temperature that are smaller than their diurnal variations. In addition, large bodies of water, such as the ocean and very large lakes, have moderate temperature variations. The waters below the ocean surface are particularly stable in temperature.